Tuning is a very interesting subject, on which volumes have been written. Technically speaking, the only thing in tune on a conventionally tuned piano are the octaves. All the other intervals are stretched, so to speak, and basically "out of tune". The major thirds, especially, are audibly sharp. It's the system known as equal temperment, which allows a pianist to change keys at will. But that's a fixed pitch instrument, which means the player cannot alter the tuning in any real way during performance.
Horn and stringed instrument players have more leeway, and can find notes that don't exist on the piano. Miles does this on his solo to Freddie Freeloader, where the minor seventh interval he plays over the "four" chord is quite a bit flat from the pianist's m7. But is doesn't sound out of tune, because it's actually the 7th partial of the harmonic series, and so we respond to it as being "in tune". I know that's a more technical answer than what you were looking for, but I'm just making the point that there are quite a few different tuning systems, some of which can coexist if the musicians know what they're doing.
Aside from that, there are some who play out of tune momentarily for effect, and others who just don't play (or sing) in tune, I guess, because they haven't learned how (like most Americans singing Happy Birthday in a restaurant ). I was listening to some Mclean the other night, and it didn't strike me as being that far out.
...Many people think Von Freeman sounds out of tune.